1. Field of the Invention
The present invention relates to a transmitting circuit for use with a mobile communication terminal unit, and in particular, to that using plural transmission frequency bands.
2. Description of the Related Art
In the art of mobile communication, as the number of terminal units that are used is increased at an explosive pace, the traffic is also drastically increased. Thus, there are plural systems whose frequency bands are far apart. In addition, to improve users' convenience, there are market's needs of which one terminal unit can use the plural systems.
In such a background, it is preferred to commonly use a part of circuits for plural frequency bands that are far apart, not mount all these circuits, from view points of cost and mounting area. This point is clear because there are market's needs in which terminal units tend to become small.
FIGS. 1 and 2 show examples of structures of conventional transmission power amplifier circuits that can transmits signals of plural frequency bands. FIG. 1 shows an example of the structure of which a first stage amplifier and a second stage amplifier commonly use two frequency bands. FIG. 2 shows an example of the structure of which circuits for two frequency bands are independently disposed.
However, in the example of the structure shown in FIG. 1, although the number of structural parts is small, when the two frequency bands are far apart, it is very difficult to assure high efficiencies of a first stage amplifier 11 and a second stage amplifier 12. In addition, characteristics such as distortion of one frequency band are remarkably restricted. Moreover, depending on the structures of the first stage amplifier 11 and the second stage amplifier 12, it is impossible to decrease the levels of unnecessary radiation waves with last stage amplifiers 13 and 14.
Unlike with the structure shown in FIG. 1, with the structure shown in FIG. 2, efficiencies and various characteristics of first stage amplifiers 21, 31 and second stage amplifiers 22, 32 can be optimized in their frequency bands. However, in this case, semiconductor dies for the first and second stage amplifiers and a space for matching circuits disposed upstream thereof are necessary in proportion to the number of frequency bands. Thus, this structure is not suitable from view points of cost and size.
In other words, when the conventional power amplifiers uses plural transmission frequencies, plural circuits having the same structure are disposed at the sacrifice of circuit scale. Alternatively, a part of circuits is commonly used at a sacrifice of efficiency and distortion of particular frequency band.